Polyolefin polymers are well known and are useful in many applications. In particular, linear low-density polyethylene (LLDPE) polymers possess properties that distinguish them from other polyethylene polymers, such as branched ethylene homopolyethylene polymers (low density polyethylene, LDPE). The market for LLDPE grown rapidly, particularly for applications such as blown and cast films, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. A principal area for LLDPE copolymers is in film forming applications because copolymers typically exhibit high dart impact, high Elmendorf tear, high tensile strength and high elongation, in both the machine direction (MD) and the transverse direction (TD), compared with counterpart LDPE resins. Some of these properties are described in U.S. Pat. No. 4,076,698.
Ziegler-Natta type catalyst systems for the producing polyethylene and linear low density polyethylene (LLDPE) are well known in the art. An example of such a catalyst system is described in U.S. Pat. No. 3,113,115.
Recently, advances in polymerization and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties and that are useful in a wide variety of superior products and applications. Areas of improvement in olefin polymerization catalysts are exemplified by improved co-monomer incorporation, narrower molecular weight distribution, uniform particle size distribution and copolymer composition distribution, and efficient control of the average molecular weight and morphology of the catalyst particles for the heterogeneous co-polymerization of olefins, especially ethylene.
With the development of new catalysts, the choice of available polymerization techniques (solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Also, advances in polymerization technology have provided more efficient, highly productive and economically enhanced processes. Especially illustrative of these advances is the development of technology utilizing metallocene, non-metallocene catalyst systems, and other advanced “single-site” catalyst systems.
Using metallocene and metallocene type catalyst precursors for alpha-olefin polymerization affords better control of molecular weight and provides narrower molecular weight distributions for the resulting polymer, relative to classical Ziegler-Natta catalyst components composed of a titanium trichloride or titanium tetrachloride and a trialkyl-aluminum as co-catalyst. Metallocene and metallocene type catalysts and their polymerization processes are well known and studied in the art. These catalyst precursors and metallocene-based polymerization processes are, however, limited in many respects of commercial applications. Metallocene type catalysts typically exhibit relatively moderate activity for alpha-olefin polymerization, and are limited in terms of availability and versatility for current polymerization process. More importantly, metallocene catalyst precursors are relatively difficult to synthesize, a fact that limits the possibility of developing new varieties of metallocene type alpha-olefin polymerization catalysts.
Currently, there is growing interest in developing non-metallocene catalyst precursors, catalyst systems, and related catalytic processes for alpha-olefin polymerization to produce polyolefin with well-defined bulk physicochemical properties and molecular physicochemical characteristics. Methods are being sought to overcome the above-mentioned limitations associated with metallocene catalyst systems. One example of a non-metallocene catalyst precursor is activated with methylaluminoxane (MAO) as co-catalyst, and is preferably employed for catalysis in an aromatic hydrocarbon solution. Unfortunately, this catalyst system is not suited for heterogeneous polymerization in aliphatic hydrocarbons or for gas phase polymerization.
Catalyst systems in industrial slurry or gas phase processes typically comprise a catalyst compound immobilized on a carrier or support material such as silica or alumina. Supported or heterogeneous catalysts increases process efficiencies by allowing the forming polymeric particles to achieve shapes and densities that maximize reactor operability and ease of handling. Examples of supported metallocene catalyst system for industrial slurry or gas phase polymerization are described in U.S. Pat. Nos. 6,524,988; 6,521,728; 6,469,113; 6,444,606; 6,432,860; 6,420,501; 6,433,111; and 5,439,995. An example of a supported non-metallocene catalyst is described in U.S. Patent Application No. 20020161141 A1. This application describes a polymerization process whereby an unsupported non-metallocene catalyst solution and a slurry of silica-supported methylalumoxane (MAO) is introduced into the polymerization reactor.
However, bulky ligand metallocene and metallocene-type catalysts, non-metallocene-type catalysts, and even ‘single-site’ advanced catalysts typically exhibit lower activity when supported compared to the activity of a non-supported or homogeneous form. This “support effect” makes commercialization of these promising catalyst systems more difficult in existing polymerization process. Consequently, there is a need in the art for method of modifying a Ziegler-Natta catalyst with non-metallocene ligands to form a solid catalyst component with high activity.